It is generally known that in a paper machine the backing fabrics, the cylinders of a drying section and a paper web to be dried carry along a lot of air. This creates positive or negative pressures at the junctions between backing fabrics, cylinders and web. This results in the unstable running of a web, e.g. for example, a web to be dried tends to rise off a backing fabric causing twisting, flapping etc., and the like of the edge of a paper web, which is a major problem especially at high running speeds resulting in the tearing hazard of a paper web. This problem appears both during normal operation and during the lead-in of the paper web.
Patent literature discloses a plurality of solutions for overcoming the above problems. The main object in these solutions is to effect the control of pressures prevailing at the points of engagement and disengagement between cylinders and webs running thereover as well as backing fabrics by means of various air control boxes which are complicated in construction. This type of boxes have been described e.g., for example, in U.S. Pat. No. 4,441,263 and in Finnish Patent 72547.
For stabilizing the running of a web on the cylinder jacket itself it has been proposed that the cylinder be provided with suction which is restricted to a certain suction zone only, i.e. that is to that sector in which the web travels along the jacket of a cylinder. Such arrangement has been disclosed e.g., for example, in Finnish Publication 72162. In this case, suction has no effect on the behaviour of a web upstream of the cylinder jacket.
U.S. Pat. No. 3,868,780 discloses a drying section in a paper machine, wherein the part of a cylinder group facing the backing wire of a web to be dried is designed as a closed space which is in communication with this suction. This closed space is quite large and such arrangement is not capable of sufficiently controlling the pressure at various points with suction effected being distributed rather non-uniformly over the area of a group of cylinders.